He felt he had stopped and looked enough to see the road was clear. However, he had treated the junction more like a YIELD and the markings and signage clearly said STOP.
A STOP requires the driver to bring the car to a complete stop for at least 3 seconds. Remember, the STOP has been put thereinstead of a YIELD for a reason - perhaps limited visibility to either right, left or both, or coming from a minor road onto a busier and faster main road. How do you count 3 seconds? Best way - put on the parking brake, look right and left and then move off if safe - this will take a minimum of 3 seconds.
Also, the parking brake prevents the car going out onto the road in the case of the driver losing concentration and bringing the foot off the clutch, or, another vehicle shunting the car from behind. The driver is also free to take the foot off the footbrake and onto the gas, getting the biting point ready to move off.